In the field of fetal and infant development, a deformation is specifically a defect that occurs when normally developing tissues (bone, muscle, etc.) respond appropriately to abnormal mechanical forces.
This is in contrast to malformation, in which refers to abnormal morphogenesis (i.e., there is no mechanical force causing the problem), and disruption, which refers to the breakdown of previously healthy tissue. The term dysplasia may also be used when refer to tissue defects that are caused on the cellular level.
Deformations are often the result of problems in the womb, partially because that is where so much of a child's growth takes place, and partially because it is very hard to know if a fetus is sitting wrong.
Many deformations are due to fetal constraint, in which some pressure or barrier is acting on the fetus, perhaps due to an ectopic pregnancy, the presence of a twin (or three), or oligohydramnios. These factors tend to especially affect the development of the feet (e.g. club foot) or facial/cranial development. Deformations can also be caused postpartum, e.g., infants habitually sleeping with their head always turned to the same side can develop torticollis and plagiocephaly. These sort of causes are often referred to as a group as extrinsic, and are often more localized or limited in their impact.
Deformations can be secondary to other types of defects. For example, a neuromuscular imbalance resulting from a malformation might result in decreased fetal movement, which in turn leads to deformations of the joints. These sort of deformations are referred to as intrinsic, and are more likely to be systemic and comprehensive in their impact.
Some congenital syndromes have been identified as deformation sequences. That is, an initial deformation results in a chain reaction presenting a common group of symptoms. One example of this is the oligohydramnios sequence (AKA the Potter sequence or Potter's syndrome), in which oligohydramnios (insufficient amniotic fluid in the womb) results in a polycystic kidney, bilateral renal agenesis, obstructive uropathy, atypical facial appearance, limb malformations, and severe respiratory insufficiency.
As the body slows its growth and settles down, deformations become less common. In older populations, particularly geriatric populations, the terms deformity and deformation are used in a more general sense to refer to a dysfunctional structural change, e.g., a spinal deformity may be defined simply as "a curvature in the spine where the alignment is outside of defined normal limits".